And I don't think the Quizzy being one and the same as a Faith Spirit would cheapen their character in the slightest.
I think the Quizzy just is special to begin with.
This is what i tried to express with my thumbeling english skills

About Mythal/Solas, I don't see a romantic connection between them. Maybe a familial one, or a very close friendship. He admired her greatly for caring about her people. He believed that she didn't take advantage, or misuse her people. She treated them with compassion, and gave them justice when it was needed. The fact that the rest of the Evanuris killed the one he considered the best of them was Solas's breaking point.
As far as the scene with them at the end of the game, I think it was something they had planned long ago. I suspect Solas needed her help to deal with the Evanuris when he removed the veil. Because when you talk to him about it, he says he has a plan. Flemeth/Mythal never did anything before without a purpose, and that scene feels like it was planned between them. She didn't seem surprised, or betrayed, or anything about it. And if Mythal is truly all about justice/vengeance, she would have reacted in a way that showed it if he had betrayed her.
I think a previous romance between the two would be inappropriate, because Solas is still stuck in the past. He has never moved on. The devs wouldn't have made him a romance option if that was the case, because it does cheapen his relationship with Lavellan. And we know for a fact that he loves Lavellan with all that he is, and it's killing him to stay away from her.
I didn't really see a romantic connection either. I just wanted to express why it would bother me because apparently it doesn't matter to some people
How is this possible? This would break me apart if it was true.
I don't know if I remember it correctly but didn't romanced Lavellan or any Inquisitor for that matter, ask who Mythal is to him and he interrupts before she finishes to say.....

This scares me! 
Especially cause how Lavellan seems to suspect it or something how she says " or is she..." (something to that nature, I don't remember the exact dialogue.